The present invention relates to a rangefinder for obtaining information about the three-dimensional (3D) location of an object and also relates to an imager.
FIG. 22 illustrates an exemplary configuration for a prior art rangefinder. As shown in FIG. 22, a light source section 10 includes first and second light sources 11A and 11B. Filters 12A and 12B are disposed in front of the light-emitting ends of the light sources 11A and 11B, respectively. The light beams that have been emitted from the first and second light sources 11A and 11B are combined into a single beam at a half mirror 13. Then, the combined beam is projected onto an object 1 after having passed through a slit 14 and a rotating mirror 15. The output wavelengths of the light sources 11A and 11B are defined within the infrared range of the spectrum.
FIGS. 23(a) and 23(b) illustrate exemplary characteristics of the filters 12A and 12B. As shown in FIG. 23(a), the filters 12A and 12B may selectively transmit light beams with mutually different wavelengths. Alternatively, these filters 12A and 12B may separate the light in accordance with the wavelength.
A camera section 20 includes first and second imagers 22A and 22B for measuring the distance of the object 1. In front of the light-receiving ends of these imagers 22A and 22B, disposed are filters 23A and 23B, which exhibit the same characteristics as the filters 12A and 12B in the light source section 10, respectively. By using these filters 23A and 23B, the imagers 22A and 22B can separately receive respective parts of the light beams that have been emitted from the first and second light sources 11A and 11B and then reflected from the object 1. The camera section 20 further includes a third imager 22C for receiving light in the visible range of the spectrum. The output signal of the imager 22C is processed by a color signal processor 27, thereby obtaining a texture image (or color image) of the object 1.
FIG. 24(a) illustrates a relationship between the intensity of the projected light beams and the projection angle θ of the combined light beam. As shown in FIG. 24(a), a light source controller 16 controls the intensities IA and IB of the light beams emitted from the light sources 11A and 11B, respectively, as the projection angle θ of the combined light beam is changed by the rotating mirror 15. Consequently, the intensity ratio IA/IB changes as shown in FIG. 24(b). As can be seen from FIG. 24(b), there is one-to-one correspondence between the intensity ratio IA/IB and the projection angle θ. That is to say, if the intensity ratio IA/IB is known, then the associated projection angle θ is identifiable immediately. In addition, once the projection angle θ has been identified, the distance Z to the object 1 can be obtained as shown in FIG. 24(c).
Hereinafter, it will be described how the prior art rangefinder shown in FIG. 22 operates.
First, in the light source section 10, the first and second light sources 11A and 11B output respective light beams. These outgoing light beams pass through the filters 12A and 12B, respectively, and then combined into a single beam at the half mirror 13. Next, the combined light beam is transformed into vertically elongated slit-like light beam at the slit 14. Then, the slit-like light beam is reflected from the rotating mirror 15, which is controlled by a rotation controller 17, so as to be projected onto the object 1.
The reflected part of the light beam incident on the object 1 enters the camera section 20. The respective imagers 22A, 22B and 22C receive the reflected light beam via a lens 21 and half mirrors 24A and 24B. In this case, the combined light beam is separated by the filters 23A and 23B into respective beams, which are subsequently incident on the first and second imagers 22A and 22B. Each of these beams separated has a single corresponding wavelength.
A first light source signal processor 25 receives the output of the first imager 22A and outputs a video signal corresponding to the reflected part of the light beam that was initially emitted from the first light source 11A. In the same way, a second light source signal processor 26 receives the output of the second imager 22B and outputs a video signal corresponding to the reflected part of the light beam that originated from the second light source 11B. Responsive to the video signals provided from the first and second light source signal processors 25 and 26, a range calculator 30 calculates the intensity ratio on a pixel-by-pixel basis, and then estimates the projection angle θ for each pixel based on the correspondence shown in FIG. 24(b).
In this case, a viewing angle ø is defined for each pixel location at the imager as an angle formed between a line of sight passing through the center of the lens 21 and the optical axis of the lens 21 as shown in FIG. 22. There is also one-to-one correspondence between each pixel location and the associated viewing angle ø. Thus, if a particular pixel location is given, then the associated viewing angle ø is known automatically. In addition, the distance D between the center of the lens 21 and the center of rotation of the rotating mirror 15 is also already known as shown in FIG. 22.
Thus, the range calculator 30 can obtain, by the triangulation technique, the distance Z between a point on the object 1, which corresponds to each pixel location, and the camera section 20 on a pixel-by-pixel basis by substituting the projection angle θ, viewing angle ø and distance D into the following Equation (1):Z=(tanθ·tanø/tanθ−tanø)·D  (1) In this manner, information about the 3D location of the object 1 can be obtained.
Also, not just the information about the 3D location of the object 1, but the texture image (or color image) of the object 1 are obtainable from the color signal processor 27 based on the output of the third imager 22C.
The prior art rangefinder, however, has the following drawbacks.
Firstly, in the conventional rangefinder, the optical output power of the light source section 10 is supposed to be adjusted by the user. In fact, the power has been adjusted appropriately through the user's experience or by his or her trial and error. In other words, if a beginner should handle such a rangefinder, it is usually difficult for him or her to control the optical output power effectively. As a result, the user cannot always obtain precise range information. Secondly, if the object is on the move, then the optical output power should be re-adjusted for every movement of the object, thus taking too much time and trouble. Thirdly, supposing the output power is set too high while the object is located very close to the rangefinder, some harm might be done on the object.
Moreover, the known rangefinder has additional problems. FIG. 25 illustrates the signal levels of a video signal during a 3D imaging process. In FIG. 25, L represents the signal level of the overall video signal, LA represents the signal level of reflected part of the light and LB represents the signal level of an image component of an object (i.e., background light). To increase the precision of the 3D location information, the optical output power of the light source section 10 should be set at such a value as increasing the signal-to-noise ratio of the signal level LA of the reflected light component. However, since the dynamic range of the camera section 20 is predefined, the signal level LA of the reflected light component cannot be increased above a certain maximum acceptable level. On the other hand, the signal level LB of the image component of the object should be no lower than a certain minimum acceptable level to obtain a normal two-dimensional (2D) image. Accordingly, the signal-to-noise ratio of the object cannot be increased sufficiently, thus interfering with the precision improvement of the 3D location information.
Furthermore, in the field of computer vision, for example, a technique of dividing the image of an object into foreground and background parts based on the range information about the object and then separating only the foreground part is known. However, a system like a videophone is strongly required to separate a human face image (i.e., the foreground part of the image) from the background accurately, but it is not always necessary for such a system to obtain the range information itself about the object. Accordingly, a technique of dividing the image of an object into foreground and background parts without using the range information thereof should preferably be developed.